This invention relates to computer printers, and particularly to feed mechanisms and methods for feeding media from a stack.
For feeding media from a stack for sequential printing of sheets, it is necessary to individuate the sheets. This may be done by various means, including systems using friction to drag a top sheet from a stack to a media transport system and printing system. However, such systems relying on friction such as provided by rubber rollers can be unreliable as the surface of the roller wears. In addition, the rubbing effects can impair printed output, particularly for specialized coated media used for high quality photo reproduction.
One effective means to individuate a top sheet that is not subject to degradation from wear, and which does not rub on sheet surfaces is a xe2x80x9cwave drivexe2x80x9d system. Wave drive systems operate by repeatedly rolling a roller along the surface of the top sheet in a stack in a given direction. This shifts each upper sheet relative to the sheet on which it rests, with the shift being greatest at the top of the stack. With one sheet shifted laterally away from the others by an adequate margin, it encounters the nip of a pair of feed rollers, and is drawn individually from the stack.
The rolling effect relies on the inherent air space between and compressibility of the sheets, and involves no friction with or among the sheets, other than the minimal static friction of a freewheeling roller in contact with the top sheet surface. A wave drive mechanism typically has several rollers that each are moved about an oblong track over the sheets, so that motion on the lower run of the oblong is in a straight line while in contact with the sheet, and so that a next roller on the track contacts the sheet approximately when or before the prior roller departs the sheet to return via the upper track for another cycle. Each cycle shifts the top sheet slightly relative to the next sheet, and multiple cycles are required.
While effective for many applications, wave drive systems have a limitation for high speed printing. The speed of motion of the top sheet does not remain proportionate to the speed of operation of the wave drive mechanism over the course of printing a stack of media. Initially, with a thick stack, the significant compressibility of the stack makes the wave drive mechanism reasonably efficient, so that a sheet is moved at an adequately fast velocity for a given speed of drive operation (i.e. the velocity at which the axes of the rollers translate with respect to a fixed frame of reference.) However, as the stack becomes more depleted, the efficiency drops, and the rate of wave drive operation may limit printing speed. This occurs when, after a sheet is taken by the feed roller nip, the wave drive mechanism takes too much time to advance the next sheet to the nip, so that the leading edge of the next sheet lags excessively behind the trailing edge of the prior sheet, effectively reducing printer throughput rate.
Moreover, different sizes, types, and thicknesses of media respond differently to a wave drive mechanism, and each has a different function of feed efficiency versus stack height. Even if the characteristics of each type were known, and a separate program implemented to increase the speed of the wave drive mechanism, it would be impractical to look up and enter the media type for each job, even assuming that the user was aware of the need and remembered to do so. And if one selected a wave drive operation speed adequate to move the least efficient media type at a nearly depleted condition, the speed would be so high that the mechanism would generate unwanted noise, and be subject to needless premature wear or failure.
The present invention overcomes the limitations of the prior art by providing a printer with a media feed apparatus having a media tray with a surface for supporting a stack of media sheets. A wave drive mechanism above the stack support surface has a media contact element operable to make contact with a top sheet of a media stack in the tray while moving in a feed direction to shift the top sheet in the feed direction. An encoder above the tray operates to detect motion of the top sheet. A controller may monitor the encoder to determine the speed of sheet motion, and set the speed of the wave drive mechanism to maintain the sheet motion above a threshold.